Chromium-aluminium steel adapted to be used for articles exposed to high temperatures



CHROMIUM ALUMINIUM STEEL ADAPTED TO BE USED FOR ARTICLES EXPOSED TO HIGH TEMPERATURES Filed NOV. 12, 1931 June 9, 1936. E SCHEIL 2,043,631

uPPER LIMIT OF THE SCALE-PROOF RANGE AT 900 AN D I200 0 4 6 a 10 12 14 CR INVENTOR.

Schez'] ATTORNEYS.

Patented June 9, 1936 PATENT OFFICE K 2,043,631 CHROMIUM-ALUMINIUM scram. "Amman TO BE USED FOR ARTICLES EXPOSED TO HIGH TEMPERATURES Erich Scheil,

Dortmund, Germany, assignor to the firm Vereinigte Stahlwerke Aktiengesellschaft, Dusseldorf, Germany Application November 12, 1931, Serial No. 574,628

v In Germany November 29, 1930 Claims. (Cl. 75-124) This invention relates to chromium-aluminium steels adapted to be used for articles exposed to high temperatures.

The present'invention relates to chromium- .aluminium steels of special composition, and

especially to their employment for articles that are to be exposed to high temperatures and must therefore be free from tendency to'scale.

In the ordinary sense, the term "scale proof is applied to steels the surface of which does not, as is the case with acid-resisting steels, remain perfectly bright, but nevertheless forms only a very small amount of, scale, this latter, moreover, soon diminishing, as the exposure is prolonged, owing to the formation of a special superficial film, so that the attacking action practically comes to a standstill after a short time.

This requirement in respect of non-scaling properties is only partly fulfilled by the scale formed, on pure ferro-chromium alloys containing up to about 30% of chromium and consisting substantially of F6304, even when the substitution of chromium for iron increases. In

such case, even slight additions of aluminium do not produce any alteration at first, since the form of the black, velvety, lustrous Fe3O4 is retained even when part of the iron is replaced by chron'iium or aluminium.

If, however, the additions of aluminium be increased, the black scale is accompanied by a whitish scale consisting chiefly of alumina, and whichdoes not form any mixed crystals with F6304. Experiments have shown that this whitish scale protects the steel, to a very considerable extent, from further attack. The efficacy of the white scalebecomes particularly manifest when a range of concentration is maintained in which black scale is formed concurrently with the white scale, the parts covered with the black scale being somewhat extensively attacked, so that thesev steels, cannot be ranked as particularly scaleproof. Reliable protection is not attained until sufficient aluminium is added for a pure white scale to be formed. As may be gathered from the accompanying diagram, a very considerable improvement in scale-proof properties is obtained when a certain'proportion (about 8%) of aluminium is exceeded, even with relatively low proportions of chromium.

According to the results of experiment, a-certain relation exists between the minimum amount of aluminium that must be employed to produce an effective scale-proofing, and the chromium content which must be employed in conjunction with the said aluminium content. Whereas with an aluminium content of 5%, chromium to the extent of 6% is needed, this content becomes 4% when the aluminium is increased to 6.5%, falling to 3% with an aluminium content of 8%, to 2% 5 with an aluminium content of 9%, and to 1% of chromium with an aluminium content of 11%.

Accordingly the present invention comprises objects-which are exposed to high temperatures up to about 1100 C. consisting substantially of 10 a steel alloy containing chromium in amounts between about 1% and 6%, aluminium in amounts between about 11% and 5% the sum of chromium and aluminium amounting to between about 10% and 12%, the balance of the alloy consisting pre- 15 dominantly of iron, said alloy showing. under the action of gases containing oxygen a coating of a white oxide, the so-called white scale. It is to be undnerstood that small quantities of other substances such as will not-injuriously affect the nature of the alloys are comprised in the claims by the term the balance consisting predominantly 'of iron.

In order that the steel may be forgeable, the carbon content must be kept below 0.1%. Alloys with this carbon content can be worked without any great dimculty.

It is important-that the occurrence of an excessive amount of slag should be avoided in the production of the alloys. If the slag content is relatively high, black scale, in the vicinity of which the steel is attacked in a manner resembling the formation of cracks, forms in certain places; It has been ascertainedthat this phenomenon does not occur when the steel is melted in high-frequency induction furnaces, so that the method of production is an important factor in the preparation of scale-proof alloys.

Up to about one half of the aluminium content may be replaced by silicon, be ium, titanium or vanadium, and up to about 50% of the-chromium content may be replaced by tungsten,

'nickel, molybdenum, manganese or cobalt. Al.

though these modifications cause a variation in 5 the limits of the scale-proof range, the substan tial point, even when the said metals are employed, is to remain within the white-scale range.

I claim:

1. Objects which are exposed to high tempera; tures consisting substantially of a steel alloy containing about 4% chromium and.6.5 up to 11% aluminium and the balance substantially all iron.

2. Objects which are exposed to high temperatures consisting substantially of a steel alloy conchromium and 6.5%

up to about 900 taining up to about 0.1% carbon and about 4% up to 11% aluminium and the balance substantially all iron.

3. Objects which are exposed to temperatures up to about 900 C. consisting-substantially of a steel alloy containing chromium in amounts between about 1% and 6%, aluminium in amounts between about 11% and 5% the sum 01' chromium and aluminium amounting to between about 10 to 12%, the balance of the alloy being substantially all iron, said alloy showing under the action or gases containing oxygen a coating or a white oxide, the so-called white scale.

4. Objects which are exposed to temperatures 0. consisting substantially 01 a steel alloy having a carbon content below about 0.1% and con chromium in amounts between about 1% and0%, aluminium in amounts between about 11% and 5%, the sum 01' chromium andaluminium mounting to between 10%; and 12%,the balance of the alloy being substantially all iron, said alloy showing under the action oi containing oxygen a coating of a white oxide, the so-called white scale.

5. Analloy of iron, chromium and aluminum in which the aluminum content is from 9 toIl percent. and the chromium content is from 2 to i 1gebrcent, "and the balance being substantially all 

